by Amy Lyn Edwards
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Introduction:
They have successfully occupied a wider range of habitats than almost any other group of organisms. They have solved difficult problems of living in an amazing variety of ways. In the process of expanding their ecological horizons, they have diversified into about 100,000 living species.
Practical interest in mollusks as food for man and as objects incorporated or depicted in art and religion far predates any scientific study. People engaged in molluscan commerce long before the dawn of recorded history. Seashells were traded from tribe to tribe, ending hundreds of miles from the ocean as offerings in funeral mounds. Kitchen middens left by seashore dwellers, heaps of empty land snail shells found in archaeological digs in, and hill sized mounds of river mussels eaten by Indians testify as to how long mollusks have been part of man's diet. Art objects from cultures in all parts of the world have incorporated shell motifs into designs. Both ancient Phoenicians and Amerindian tribes in Western Central America used glands from muricid snails to prepare a brilliant purple dye, known as Tyrian Purple.
For thousands of years seaside visitors have been enchanted by the shapes and colors of shells washed up by the tides or storms. The antiquity of shell collecting as a hobby is suggested by the find of some shells in the ruins of Pompeii, the city buried by an eruption of Mt. Vesuvius in 79 A.D. Admired for their beauty, often coveted for their rarity, and nearly infinite in their variations, shells of mollusks have been and remain one of the most popular objects to collect.
In spite of this popularity, most people do not realize that these shells were made by animals. These animals affect man in many ways. Gourmets treasure the slippery feel of a raw oyster. Countless canaries have pecked at cuttlefish bones. Shell jewelry is worn daily. Ground fossil clam shells become lime for chicken feed or are scattered as road gravel. Small spheres of River mussel shells are inserted into the bodies of pearl oysters off the coast of Japan, to be returned a decade later inside a cultured pearl. Tiny snails gliding on the muddy bottoms of Mediterranean rice patties can harbor parasites that will later infect man. Gardeners the world over mutter about the damage done to ornamental plants by snails and slugs.
Despite their mostly inconspicuous ways, mollusks play an important and large role in nature. Mollusks on land play a major role in recycling organic matter back into simple chemicals that can then be reused by plants. In both fresh and ocean waters, clams are major factors in maintaining water purity. They filter out and consume microscopic organisms and particles of organic matter that otherwise could accumulate in huge quantities. Clams, in turn, serve as primary food sources fort many fish and birds. The burrowing habits of the clams help to turn over the sediment and aerate it.
Molluscs and the myriad other species of organisms living today are the latest products of more than 3 billion years of evolutionary change. Interactions with both the physical environment and the countless other individuals living in the same area go on today. The slow evolution of life proceeds.
Shell structure:
The structure of the molluscan shell complex, and the ways in which it can be modified are numerous. Basically it consists of an outer covering, the periostracum, which is composed of organic chemicals and several inner layers of calcium carbonate. Although similar in appearance to the 'chiton' of an arthropod shell, the periostracum is chemically distinct and is usually referred to as 'conchin' or 'conchiolin'. This provides some protection to the underlying calcareous shell layers against erosion or boring organisms. The periostracum also may serve to form much of the microscopic shell sculpture.
The underlying calcareous shell structures are complex and variable. The calcium carbonate crystals are laid down in an organic matrix, deposited either as calcite or aragonite. Calcite is more stable, it is the main shell substance in oysters. Aragonite is the major component of freshwater and land snails. In many marine molluscs the layers of calcite and aragonite alternate.
The brilliant 'mother-of-pearl' inner surface of shells like the abalone, is made up of very fine layers of aragonite. Approximately 450 to 5000 layers for each 1/25 of an inch thickness is layed down. The crystals are deposited horizontally producing a polished surface. The iridescence in the mother-or-pearl layer is caused by the physical interaction between the crystals and light waves.
The Animals:
The phylum Mollusca is characterized by a highly flexible body plan. It's few elements are combined and greatly distorted to achieve very different ways of living and methods of locomotion. A mollusk can be thought of as consisting of a body mass (visceral hump) covered with protective armor (shell) from which extends sensory-feeding (head) and locomotion (foot) areas. They are altered and combined in the various ways that form the confusingly different patterns of the major molluscan groups.
Two anatomical structures are unique to the phylum Mollusca, the mantle and the radula. The mantle is a flap from the visceral hump that extends out and downward, forming a cavity (mantle cavity) between its outermost edge and the visceral hump. The mantle tissue secreates the material that becomes the mollusc's shell. The radula resembles a toothed tongue. Depending on the species, the radula can have a few teeth to many millions of teeth. In most snails, the radula is moved back and forth over a surface to scrape off food. the part of the radula that has old worn teeth is continually being re-absorbed, while the other end has new teeth added to it. The following pages briefly describe the 5 major molluscan classes:
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